How to Test a Thermostat with a Multimeter!

In this HVACR Video, I go over how to Test and Read Voltage to Verify if a Thermostat is Working Properly. I use a Multimeter in order to read each 24v signal at the control board. I go over the thermostat color code and terminal functions as well as how the thermostat works in order to show how to diagnose the thermostat. Supervision is needed by a licensed HVACR Tech while performing tasks as Experience and Apprenticeship garners Wisdom and Safety.

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[kennyw]

I had a service manager at Trane that used to always say "back to basics" and this reminds me of that. Great video.

[Karl-ejrs]

Thank you so much for this video. I had 28 volts at the unit but only 17 at the thermostat so my thermostat wouldn't run unless it had batteries in it. When I looked at the wiring the builders never hooked up the common wire inside the unit that runs to the thermostat. Once I connected that it fixed everything!

[tedlahm]

Wonderful video. An oldie but goodie.

[jackpast]

Nice video, Craig. Those mag jumpers are pretty cool.

[arulmoviemakers]

Thank you! My replacement thermostat was expecting 'C' connection, but my existing thermostat did not have C, but there was wire available, but not connected. I opened the HAVC and connecting the 'C' wire, and connected the new thremostat- but it was not working - I though i blew the fuse. After seeing your video, I realized it is the auto-shutoff safety switch!! once i put the panel back on it started working again. Thank you very much!

[brianshanahan]

One of the best I've seen! My problem is that 24VAC no longer present between R and C...

[wendyyu]

Thanks for the video. Question: how to measure the volts if there is no C wire?

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If you want to support the ACSERVICETECH Channel and receive more HVACR content, check out www.Patreon.com/acservicetech

[tedlahm]

Calling for cool, Red to yellow, turns on the fan WITHOUT jumping to the green also?

[Mspikes]

Thanks for reminding me that my panel has a door switch! I could NOT figure out why my a/c unit wouldn’t turn on after I removed it!

[ronaldcach]

Great, cleared up some gray areas I had. THANKS FRIEND

[Goodtimes]

Thank you! Great vid, it worked to trouble shoot a problem at work

[shreddagorge]

Quite helpful! I'm untrained in HVAC and thought I'd see 24 VAC between Rh and W when heating - not the W and C as you've measured (makes sense now). I have a gas fireplace (mV system) where an existing Honeywell connected Rh and W to replace what was previously a manual 'light' switch. To hook up my Sensi WiFi thermostat, I'm thinking of two options: (1) de-jumper Rc and Rh and wire Rc and C to the 24 VAC supply (for the thermostat to use), then wire Rh and W like the Honeywell it replaces. Seems easiest, but I'm not convinced the right way because there may have been a setting inside the Honeywell allowing it to support millivolt systems which my Regency P42 is. Option (2) is to wire W and C to a 24 VAC relay simulating the fireplace on/off switch. You make the point in another reply that current is flowing through Rh and W, so it sounds dangerous to use the thermostat's internal relay as a switch - I'll check the Honeywell manual if there was a setting to de-energize it on that one but the Sensi manual simply says "millivolt systems are not supported."

[williampetty]

These are very good videos. Very informative

[seattlebest]

If there's no heating or cooling, do i need to do the jumps in the thermostat only or in the control board, too? Thanks

[MonsterNinjaz]

When u put it heat mode in auto should the W and G get power or just W?

[zackvizeman]

Thank you for the great video. I have only two wires coming to my thermostat, connected to R and W. The voltage measures 77 V-AC. The thermostat is rated (like most other thermostats) at 24 AC and it has no issues. What puzzles me is how can you get voltage between a wire that is supposed to have 24 AC (attached to terminal R) and the wire going to terminal W, which I believe, is supposed to complete the circuit of the zone valve, thereby turning the boiler on?! I thought one can measure the 24 V line with the other probe attached to a common wire/terminal only!
Your comments are appreciated in advance.
Zack

[Johnslist]

Thanks! I've tested these and they are fine, my issues is that heat stops shy of set temperature. Could it be the thermostat?

[srobeck]

Wouldnt it be less confusing to test the mode its in (ie cooling), from Y to R? Then the reading is more simple (with power applied): a closed switch = 0 volts; an open switch reads = 24 V